1. Field of the Invention
This invention relates to the field of thermoelectric devices and, more particularly, to a thermoelectric device wherein the device consists of thermoelements and interconnects of unique design which maximize performance while minimizing the use of costly thermoelectric material and further results in a reduction in the number of fabrication steps.
In one embodiment of the present invention, a metallic or semi-metallic support, termed a "wafer", allows powdered thermoelement material to be processed and facilitates the bonding of these formed elements to their respective interconnection members. Additionally, novel substrate and bonding techniques are also disclosed.
Other embodiments of the present invention include unique techniques relating to thermoelement surface preparation and bonding.
Lastly, other embodiments of the present invention relate to the application of thermoelectric devices in the medical therapy and electronics thermal management fields.
2. Description of the Related Art
Conventional thermoelectric devices utilize dissimilar conductive materials subjected to a temperature gradient across their leg lengths to create an EMF or electromotive force. This EMF is proportional to the intrinsic thermoelectric power of the thermoelements employed and the temperature differential between the hot and cold junctions. Alternatively, current may be introduced into the circuit to move heat, absorbing it at one junction, moving it and dissipating it at the other junction.
It is desirable that the thermoelements be of such material that the highest EMF is developed for a given temperature differential between the hot and cold junctions. The electrical resistivity and thermal conductivity of the thermoelement in the device should be as low as possible in order to reduce both electrical and thermal losses and thus increase the efficiency.
One disadvantage of current thermoelectric devices is the high cost of the semiconducting materials, which yield the highest conversion efficiencies available. A reduction in a thermoelement's cross sectional area not only reduces material volume, but increases electrical resistance proportionately. A reduction of element leg length reduces material volume and decreases electrical resistance, but it becomes increasingly difficult to maintain a temperature differential as this leg length is decreased to the point where an impracticable heat exchange mechanism is required to remove the heat faster than it is entering the thermoelectric device. This is due to the thermal conduction characteristics of the thermoelement material. Secondly, as leg lengths are further reduced, fabrication of the thermoelements themselves becomes increasingly difficult due to the semiconductor's fragile nature.
U.S. Pat. No. 3,129,117, granted to Harding on Apr. 14, 1964, discloses a method of manufacturing a thermoelectric element utilizing hot pressing in a direction perpendicular to current flow through the thermoelement.
U.S. Pat. No. 3,182,391, granted to Charland on May 11, 1995, discloses a process for forming, in one step, a thermoelement with a metallic electrical contact at one end, which comprises consolidating the thermoelectric material and metallic contact plate within a die cavity which is then hot pressed and removed from the mold cavity.
U.S. Pat. No. 3,201,504, granted to Stevens on Aug. 17, 1965, discloses a method of molding a thermoelectric couple in which dielectric sleeve members are inserted into a mold containing a conductive bottom member, powdered dissimilar thermoelectric material is added into their respective sleeves, powdered conductor is placed on top of both thermoelements, and pressing and subsequent sintering of the entire assembly yield a solid thermocouple.
U.S. Pat. No. 3,248,777, granted to Stoll in August of 1966, also discloses a thermal and electrically insulating material in which the thermoelements are cast in cavities within this insulator.
U.S. Pat. No. 3,264,714, granted to Baer, Jr. in May of 1966, discloses a thermoelectric device in which a block is composed of thermally and electrically insulating material. This block may be cut to accept inserted thermoelements or cast from a liquid or other flowable form around the spaced thermoelements and hardened. Additionally, the interconnecting members are created by electroplating over perforated metal and the top faces of each thermoelement to create the junctions.
U.S. Pat. No. 3,400,452, granted to Emley on Sep. 10, 1968, discloses using hot isostatic pressure (even, compressive pressure in all directions) to provide metallurgical bonding between the thermoelemental material and the walls of a metal tube in which it is housed.
U.S. Pat. No. 3,554,815, granted to Osborn on Jan. 12, 1971, discloses a device consisting of a thin, flexible substrate in which "bands" of dissimilar thermoelectric material are disposed on opposite sides of the substrate and perforations within the substrate contain a metallic filler to electrically connect each thermoelement.
U.S. Pat. No. 3,601,887, granted to Mitchell on Aug. 31, 1971, also discloses the use of hot isostatic pressure to provide bonding between the inner walls of a tube and the thermoelectric material.
U.S. Pat. No. 4,343,960, granted to Eguchi on Aug. 10, 1982, discloses a device consisting of a perforated dielectric substrate in which each dissimilar thermoelement is plated, in a pattern, to portions of both faces and to the walls of each thru-hole.
U.S. Pat. No. 4,459,428, granted to Chou on Jul. 10, 1984, relates to the design and manufacture of a thermoelectric device wherein the voids between each thermoelement are filled with a ceramic compound to absorb thermal expansion. Additionally, copper plates, which will later comprise the bus bars, are soldered directly to each thermoelement end and then masked and etched to form the discrete interconnects, each bridging two dissimilar thermoelements.
U.S. Pat. No. 4,470,263, granted to Lehovee, et al on Sep. 11, 1984, relates to a peltier cooled garment in which the heat pumped by the peltier unit is dissipated to the ambient via cooling fins.
U.S. Pat. No. 4,905,475, granted to Tuomi on Mar. 6, 1990, relates to a thermoelectric based personal comfort air conditioning unit. Ambient air enters and is split to pass over both the hot and cold faces of the thermoelectric device. Depending on the desired air temperature by the user, a movable baffle will distribute the correct amounts of hot and cold air to the individual.
U.S. Pat. No. 4,930,317, granted to Klein on Jun. 5, 1990, relates to a thermoelectric based localized hot and cold therapy apparatus which includes a heat sink and possibly a fan to dissipate rejected heat.
U.S. Pat. No. 5,067,040, granted to Fallik on Nov. 19, 1991, relates to the use of thermoelectric cooling to cool computer boards within an enclosure. The thermoelectric cooling device is mounted in an opening through a partition for transferring heat out of the sealed enclosure.
U.S. Pat. No. 5,097,828, granted to Deutsch on Mar. 24, 1992, relates to a thermoelectric based therapy device comprising a heat sink and fan for dissipating heat moved and generated by the peltier devices.
U.S. Pat. No. 5,103,286, granted to Ohta on Apr. 7, 1992, discloses a simultaneous sintering and bonding of the thermoelements to themselves and to their respective interconnection members in the absence of pressure. Sintering, which is the heating of an aggregate of metal particles in order to create agglomeration, does not involve simultaneous pressure.
U.S. Pat. No. 5,108,515, granted to Ohta on Apr. 28, 1992, discloses a Bi, Te, Se, Sb thermoelemental material which is pulverized to a specific particle size and then forming a green molding which is then sintered.
U.S. Pat. No. 5,108,788 and U.S. Pat. No. 5,108,789, both granted to Rauch, Sr. on Jan. 5, 1988 disclose a PbTe thermoelemental material in which the compound is: melted, chill cast into an ingot, ground to a particle size of less than 60 mesh, cold pressed to 30-70 kpsi, and finally sintered.
U.S. patent No. 5,246,504, also granted to Ohta on Sep. 21, 1993, is nearly identical to what is claimed to U.S. Pat. No. 5,108,515.
U.S. Pat. No. 5,318,743, granted to Tokiai on Jun. 7, 1994, discloses to "presinter" a Bi, Te, Se, Sb thermoelemental material, then mold the presintered powder and sinter the resultant form also using hot isostatic pressing technology. The actual thermoelements are then cut from the sintered bulk.
U.S. Pat. No. 5,429,680, granted to Fuschetti in Jul. of 1995, relates to a nickel diffusion barrier layer coated directly onto each thermoelement end.
U.S. Pat. No. 5,434,744, granted to Fritz on Jul. 18, 1995, discloses a substrated thermoelectric device in which thermoelemental spacing is less than 0.010 inch and thermoelemental thickness is less than 0.050 inch. In addition, an improved device is claimed to have greater than 300 thermoelements and their said thickness is "approximately" 0.020 inch.
U.S. Pat. No. 5,623,828, granted to Harrington on Apr. 29, 1997, relates to a thermoelectric air cooling device for the passenger of a vehicle. A fan, blowing ambient air across both the hot and cold faces of the thermoelectric device, includes a design permitting the cold air to blow onto the passenger while the hot air is exhausted away.
U.S. Pat. No. 5,800,490, granted to Patz, et al on Sep. 1, 1998, relates to a portable cooling or heating device incorporating a thermoelectric assembly comprising: a peltier device, gel pack to interface with the user along with a fan and radiator to dissipate or absorb thermal energy from the surrounding air.
U.S. Pat. No. 5,817,188, granted to Yahatz, et al on Oct. 6, 1998, relates to a thermoelectric module comprising thermoelements whose junctions are coated with bismuth or a bismuth alloy. Additionally, a solder comprising bismuth and antimony is utilized to joint the coated thermoelements to conductive interconnecting bus bars.
U.S. Pat. No. 5,890,371, granted to Rajasubramanian, et al on Apr. 6, 1999, relates to a passive and active air conditioning system for an enclosure housing heat producing equipment. This closed hybrid system cools the air existing within the heat producing equipment enclosure housing by recirculating this air across both a heat pipe device and also a thermoelectric device which transfers the heat to the ambient air.
U.S. Pat. No. 5,981,863, granted to Yamashita, et al on Nov. 9, 1999, relates to manufacturing a thermoelement in which molten thermoelement material is rapidly cooled, powdered and hot pressed within a range of time, temperature and pressure in order to reduce grain size, and thus increase material efficiency.
U.S. Pat. No. 5,987,890, granted to Chiu, et al on Nov. 23, 1999, relates to cooling an electronic component within a portable computer using a heat pipe or peltier device to move heat from the electronic component to a thermal reservoir, such as a battery.
U.S. Pat. No. 6,023,932, granted to Johnstone on Aug. 25, 1999, relates to a portable topical heat transfer device comprising a thermoelectric unit and heat sink with a fan mounted to the warm side of the peltier device.
U.S. Pat. No. 6,025,544, granted to Macris on Feb. 15, 2000, relates to a block of metallic material into which cavities are formed and filled with thermoelement material. This material is compacted and sintered. The resultant block structure is sliced, electroplated, etched and mounted to form a thermoelectric device.
A disadvantage of the existing art is the bond strength between the typically brittle thermoelements and their interconnects. In addition, the diffusion of metallic species when these dissimilar materials are in contact must be mitigated. Lastly, the bonding structure, between thermoelement and its respective interconnect, must not itself possess a significant Seebeck Coefficient so as not to reduce the performance of the two P and N-type thermoelements.
A disadvantage to the existing cold therapy technologies incorporating thermoelectric devices as heat pumps is the means by which the pumped heat is dissipated from the hot face of the thermoelectric device. Fans and/or heat sinks are cumbersome and reduce flexibility of the therapy unit.
A current disadvantage of the current personal computer or electronics enclosure cooling art is the complexity and inefficiency of the systems resulting in high costs.